I DO wish the 12-100 was smaller and lighter, which would make it much better for what I want
The beauty of this lens is that you get PRO quality optics, weather sealing, and other features but in a super zoom which are rare to have stellar IQ. Smaller and lighter gets you the 12-200 which is even more of a super zoom, but they wanted to keep it cheaper and smaller and lighter and from the sample shots I looked at when it first came out, the photos were just OK in IQ, nowhere in the same league as the 12-100 PRO photos. I think Olympus did a tremendous job with this lens giving us that high level focal range and IQ combination while still keeping it relatively smaller.
Back to the original poster, I have only heard great things about the Panasonic 8-18mm lens which does take filters. I even thought about renting one for a trip as a local camera equipment rental place had it, but ended up not doing it. It is kind of a pricey lens at $1000 usd retail new. But I see KEH.com has a few used ones ranging from $770-$822 depending on condition.
As others have mentioned, when you get in the ultra wide angle side of things, every millimeter is noticeable. I did this one on my old 4/3rds gear comparing the 14-54mm to the 12-60 SWD lenses to show the difference only 2mm made on the wide end...
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12mm vs 14mm by
Patrick, on Flickr
and this just across my living room. Obvious when out side doing building, mountains, city scenes, etc. where everything is farther away, that extra space to either side (and top and bottom) could capture a lot of stuff. Or when shooting closer like my example, may be the difference of getting the statue, fountain, building, etc. in the frame when you simply can not back up any further. Clearly 8mm or 9mm is going to be even more dramatic.